Obesity rates in 3-5 year-old American children have increased dramatically in recent decades. Concurrent with this alarming trend, time spent by young children in preschool settings has increased and their time spent in unstructured play has decreased. Recent studies have shown that children in preschools are engaged primarily in sedentary activities. Nonetheless, few interventions designed to increase PA or decrease sedentary behavior have been evaluated in preschool children. The proposed investigation will test a multi- component intervention designed to increase physical activity energy expenditure (PAEE) in 3-5 year-old children. Twelve preschools will be randomly assigned to intervention or control groups. 360 children, in two annual waves of 180 children, will participate in the measurement protocol at baseline and after exposure to the intervention for 9 months. Child-level measures will include moderate-to-vigorous PA, sedentary behavior, and PAEE measured by accelerometry;height, weight and waist circumference;and demographics. The intervention will consist of eight components: high-quality physical education, two active recess periods per day, PA in classroom lessons, reduced sedentary time in school, reduced TV watching at home, enhancing the physical environment to promote PA, enhancing the social environment to promote PA, and ensuring adoption of organizational policies related to PA. The study's intervention coordinator will work with the preschool directors, teachers, and assistant teachers to facilitate intervention implementation by providing training, materials, and ongoing support and feedback. An extensive process evaluation will document the extent to which the intervention was implemented. Effects of the intervention on PA, sedentary behavior, PAEE, and weight status will be determined. In addition, the study will examine factors that associate with change in physical activity, sedentary behavior, physical activity energy expenditure and body mass index during a school year in preschool children. Public Health Relevance: The study will address a critical public health problem, childhood obesity, by testing a physical activity intervention in the types of preschools that millions of children attend. Information gained in this study may lead to adoption of preschool policies and instructional practices that increase physical activity and reduce the risk of obesity in preschool children.